The subject matter of the present invention relates to a method and apparatus for explosively opening a production valve of a tool string adapted to be disposed in a wellbore, and more particularly, to an apparatus and associated method which includes a frangible breakup element that is adapted to shatter into a multitude of pieces when a detonation wave passes therethrough, a support, applied to a piston, being removed when the frangible breakup element shatters, the piston moving in response to a tubing pressure or a rathole pressure when the support, applied to the piston, is removed, the production valve opening when the piston moves a predetermined distance.
A tool string, adapted to be disposed in a wellbore, often includes a production valve, a firing head, and a perforating gun. The production valve can be connected between the firing head and the perforating gun in the tool string when disposed in the wellbore, or the firing head can be connected between the production valve and the perforating gun. When the perforating gun detonates, the production valve should be open in order to create an underbalance condition and permit well fluids to flow into the production valve and into the associated tubing. One of the advantages of using tubing conveyed perforating (TCP) is the ability of shoot the perforating gun in the wellbore while the well is underbalanced. The most common way to shoot while underbalanced is to use a production valve. These valves are operated (that is, opened) downhole at the appropriate time by various means, such as by dropping a weight bar from the surface which breaks off a frangible member thereby allowing pressure from either the rathole or the tubing to work on a piston and opening the production ports of the production valve. In shallow wells, where the pressure is relatively low, a problem occasionally arises when the production valve opens, that is, a resultant pressure transient in the well causes a decrease in pressure inside the tubing string and hence at the firing head that can prevent a pressure assisted firing head in the perforating gun from functioning properly The resultant pressure transient can produce a low or negative pressure in the vicinity of a firing pin of the firing head of the perforating gun at precisely the time when the firing pin of the firing head in required to move for firing the perforating gun. Still other production valves, used in association with pressure assisted firing heads, are also operated by pressure. These valves also have inherent problems in that most rely on shear pins to retain the valve in the closed condition; however, the shear pins must shear immediately prior to the firing the perforating gun. In addition, the use of a pressure operated valve in a tool string increases the total operating pressure necessary to actuate the firing head; however, the equipment located downhole in the wellbore may not be strong enough to withstand these higher operating pressures.